The present invention relates to a multilayer ceramic electronic device, such as a multilayer ceramic capacitor, particularly relates to a multilayer ceramic electronic device having an improved permittivity and improved transverse strength.
A multilayer ceramic capacitor as a multilayer ceramic electronic device comprises an element body configured by alternately stacking dielectric layers and internal electrode layers. The dielectric layer is usually comprised of a dielectric composition wherein barium titanate is a main component and the internal electrode is comprised of Ni or a Ni alloy, etc.
The multi-layer ceramic capacitor is being broadly used as a compact, large capacity, high reliability electronic device. The number used in each piece of electronic equipment has also become larger. In recent years, along with the increasing miniaturization and improved performance of equipment, there have been increasingly stronger demands for further reductions in size, increases in capacity, reductions in price, and improvements in reliability in multilayer ceramic capacitors.
Multilayer ceramic capacitors are normally produced by stacking a paste for forming the internal electrodes and a paste for forming the dielectric layers using the sheet method or printing method etc. and cofiring.
As the electroconductive material for the internal electrodes, generally Pd or a Pd alloy is used, but since Pd is high in price, relatively inexpensive Ni, Ni alloys, and other base metals have come into use. When using a base metal as the electroconductive material of the internal electrode layers, firing in the atmosphere ends up oxidizing the internal electrode layers and therefore the firing of the dielectric layers and internal electrode layers has to be done in a reducing atmosphere by controlling an oxygen partial pressure. Also, an organic binder, etc. used for producing the multilayer ceramic capacitor is hard to be removed by reducing firing, so a resin able to be discomposed at a low temperature are used which is normally degreased under a condition that Ni is not oxidized.
As explained above, since multilayer ceramic capacitors have been made compact and improved in performance, a large number of those have come to be used in a variety of electronic devices. Installation on an electronic circuit substrate is performed by using an automatic installer called a mounter, by which it is installed on an electronic circuit substrate at a high speed. When using such an automatic installer, a large mechanical load is inevitably given on the multilayer ceramic capacitor at the time of conveying the multilayer ceramic capacitor or fixing the capacitor on the electronic circuit substrate. Therefore, when mechanical strength of the multilayer ceramic capacitor is weak, a crack or chip, etc. arises on the capacitor after mounting on the electronic circuit substrate, which leads to a serious hindrance on an operation of the electronic circuit and reliability. From the above background, an improve of mechanical strength of multilayer ceramic capacitors is strongly desired.
Note that the Japanese Unexamined Patent Publication No. 9-260203 schematically describes an improve of strength without declining insulation property of dielectric layers by setting a content of carbon contained in the dielectric layer or internal electrode layer not less than 1 ppm and not more than 5000 ppm. However, the publication only regulates a range of a carbon content contained in the dielectric layers in a general capacitor, and merely describes a wishful surmise without any logical or experimental proofs. For example, it is general that carbon of not more than 5000 ppm is contained in a dielectric layer in a normal capacitor, which does not have any critical meanings.
An object of the present invention is to provide a multilayer ceramic electronic device, such as a multilayer ceramic capacitor capable of improving a permittivity and other electric characteristics and mechanical strength even when Ni or a Ni alloy is used as an internal electrode.
As a result that the present inventors engaged in intensive studies to attain the above object, it has been found that by setting a content of carbon in an element body after firing of a multilayer ceramic electronic device not less than 1 ppm and not more than 100 ppm, transverse strength of the element body is improved and the permittivity is critically improved, and they have completed the present invention. Also, the inventors have found that it is possible to provide an electronic device easy to be produced having high reliability by maintaining the predetermined relationship between carbon amount contained in an element body after firing of a multilayer ceramic electronic device and transverse strength of the element body, and have completed the present invention.
Namely, a multilayer ceramic electronic device according to a first aspect of the present invention is characterized by comprising an element body constituted by dielectric layers and internal electrode layers alternately stacked, wherein a carbon amount in said element body after firing is not less than 1 ppm and not more than 100 ppm (preferably not less than 2 ppm and not more than 50 ppm, further preferably not less than 3 ppm and not more than 15 ppm).
Also, a multilayer ceramic electronic device according to a second aspect of the present invention is characterized by comprising an element body constituted by dielectric layers and internal electrode layers alternately stacked, wherein
relationship of a carbon amount in the element body after firing and transverse strength of the electronic device satisfies a formula below.
F=xe2x88x92A*ln X+B
(ln is a natural logarithm)
note that
23xe2x89xa6Axe2x89xa628, preferably 24xe2x89xa6Axe2x89xa627
B=350 to 400, preferably 360 to 380
F: transverse strength (MPa)
X: carbon amount (ppm) contained in element body
The above internal electrode layer is not particularly limited, but it preferably contains any one of nickel, copper and tungsten, or an alloy of them.
When using nickel and other base metal as the internal electrode layer, firing of the element body is performed under a reducing atmosphere so that the internal electrode layer does not oxidize. Accordingly, when an organic binder used for forming the element body before firing cannot be sufficiently degreased, there is a possibility that carbon remains in a sintered body. It was proved by experiments by the present inventors that strength of the sintered body is deteriorated by the residual carbon.
According to the first aspect of the present invention, preferably, a carbon amount in said element body after firing is not less than 1 ppm and not more than 100 ppm, preferably not less than 2 ppm and not more than 50 ppm. When the carbon amount to be contained is excessive, the permittivity and transverse strength declines, and insulation resistance (IR) and dielectric loss (tan xcex4) tend to deteriorate. Also, it has been proved by the present inventors that when the carbon amount is too small, the permittivity again declines. According to the first aspect of the present invention, strength of the multilayer ceramic electronic device can be sufficiently improved without deteriorating the electric characteristics.
According to the second aspect of the present invention, by setting a relationship between the carbon amount contained in the element body after firing and transverse strength to be within the above relation range, firing can be performed without causing any cracks or delaminating at the time of firing the element body, the element body after firing becomes sufficiently densificated and sufficient transverse strength required can be obtained.
In the present invention, a dielectric composition constituting the dielectric layer is not particularly limited, but a dielectric composition containing barium titanate as a main component is preferable. In the present invention, particularly preferably the dielectric layer is constituted by a dielectric composition containing a main component expressed by
[(Ba1xe2x88x92xxe2x88x92yCaxSry)]m.(Ti1xe2x88x92zZrz)O3.
The atom percent ratios, x, y, z and m in the composition of the above main component are preferably in relationships below. Namely,
0 less than xxe2x89xa60.25, preferably 0 less than xxe2x89xa60.10
0 less than yxe2x89xa60.05, preferably 0 less than yxe2x89xa60.01
0 less than zxe2x89xa60.3, preferably 0.10 less than zxe2x89xa60.20
0.998xe2x89xa6mxe2x89xa61.020, preferably 1.002xe2x89xa6mxe2x89xa61.015.
In this case, MnO as a first subcomponent is preferably contained in an amount of 0.01 to 0.5 wt %, more preferably 0.1 to 0.4 wt % with respect to 100 wt % of the above main component.
Also, Re2O3 (note that Re is at least one element selected from Dy, Ho, Er and Y) as a second subcomponent is preferably contained in an amount of 0.05 to 0.5 wt %, more preferably 0.2 to 0.4 wt % with respect to the above main component 100 wt %. In the present invention, Y2O3 is particularly preferable among the Re2O3.
Furthermore, a sintering aids as a third subcomponent wherein silicon oxide is the main component is preferably contained in an amount of 0.005 to 0.3 wt %, more preferably 0.01 to 0.2 wt % with respect to the above main component 100 wt %. The sintering aids containing silicon oxide as the main component is not particularly limited and is not limited to SiO2 alone, but it may be composite oxides, such as BaSiO3, CaSiO3, Li2SiO3.
Furthermore, at least one kind from V2O5, MoO3 and WO3 as a fourth subcomponent is preferably contained in an amount of 0.005 to 0.3 wt %, more preferably 0.01 to 0.1 wt % with respect to the above main component 100 wt %.
Al2O3 as a fifth subcomponent may be preferably contained in an amount of 0.005 to 0.1 wt %, more preferably 0.01 to 0.1 wt % with respect to the above main component 100 wt %.
The present invention exhibits particularly large effects in the case of a multilayer ceramic electronic device comprising dielectric layers having the above composition (after firing).